All living beings with a soul that are born within the Planner Scenario have Levels and Skill Levels set at birth. Levels determine the strength of an individual, and can be raised by fighting or killing another living being to accumulate enough experience to be acknowledged by a Level God. Skilled adventurers tend to have a personal Level God that they can summon at any time to raise their and their allies' levels.
Levels are not permanent, however, and can go down if a person is inactive for an extended period of time. The speed at which a level can go up or down varies depending on the person. The faster someone can level up, the faster their level can decrease. The opposite applies as well. For example, the Hero of the system levels up very slowly but never loses levels, while Rance gains levels very quickly, but loses them just as fast when he procrastinates.
While levels are the most direct measure of a person's strength, physical fitness also plays a less concrete but still very important part of it as well, with the large and robust-bodied Helmanian being on average stronger than a smaller person of the same level, and an elderly person being on average weaker than a younger person of the same level. Because of this, even a person at a low level can attain a certain amount of strength by maintaining a healthy body.
Just as every person is born with levels, they are also born with a limit known as a "Level Cap" that blocks their progress. Almost all beings within the Planner Scenario (Humans, Kalar, Monsters, etc) have a level cap in order to limit their power and help maintain a balance between individuals. Once a person reaches their level cap, they have reached their full potential and cannot get any stronger. Only key members of the antagonist race such as the Dark Lords and The Demon King do not have levels or caps. However, Gele apparently opened up a door to an alternative dimension to raise her own level at the end of Rance 03, accidentally also raising that of Rance's in the process. So it seems that they also have levels, just not obvious caps comparable to that of humans.
A level cap is set the moment a person is born, and while it can be raised through artificial means, they are generally very hard to come by and don't increase it by much.
The average person has a level cap of 5 to 10. Having a cap of 20 or above is the equivalent of being first in your class and is the ideal range for most soldiers and adventurers. Caps of 30 or above are seen only once every 10,000 people and are generally seen as the minimum range to achieve a high standing in an army. Caps of 40 or above are seen once every 100,000 people and signify a person as having the potential to become one of the strongest people in the world.
Rance is a notable abnormality in this system, as he is a "glitch" that has no level cap at all, meaning that he can theoretically level up indefinitely to become infinitely stronger. This trait has also been shown to have been inherited by each of his children.
It is important to note that a level cap does not determine how strong a person is but how strong they can be with enough time and training. While several people are known to possess caps above 40, only a small handful of them have actually been able to reach levels at or beyond 40. A prime example of this can be found in Maha Margaret, who had the highest cap known of any human at 120, but was killed long before he had any chance of reaching his full potential and was still quite weak prior to his death. In contrast, Kabachahn the Lightning has a comparatively smaller cap of 46, but is considered to be one of the strongest people alive due to being very close to his cap and making full use of his skill levels.